RFC: enabling themes

please do not for merge, is just for comparison of changes and if you really want this in the app.

- the markup element gets a semantic, laverna-namespaced class
- all other classes get removed from the markup
- all styling is applied only to that semantic class
- a theme can use style libraries (like bootstrap) by extending the semantic laverna classes.
- for a locally running app, themes could be installed via bower
- a theme-switcher component will be needed

please check if there are cases where removing classes would break functionality. would be cool if you could comment those in the component markup.

libraries which couple functionality and styling to the same classes or ids would be "bad" in that regard.